1. Field of the Invention
The present invention relates generally to a laser device and particularly to a group-III nitride semiconductor laser device.
2. Description of the Related Art
In order to achieve an operation as a laser, an optical laser cavity comprising a pair of reflecting mirrors is required. In semiconductor laser devices (Fabry-Perot type) using such semiconductor crystal materials as GaAs, a multi-layer body including a laser structure is cleaved simultaneously with a GaAs crystal substrate and then this cleaved facet is used as a cavity mirror of the laser device.
As concerns a semiconductor laser device formed from group-III nitrides, sapphire is commonly used as the substrate. Since sapphire does not have a clear cleavage plane, the cavity mirror can hardly be formed by cleaving. Accordingly, the cavity mirror of the semiconductor laser devices is formed by dry etching such as reactive ion etching (RIE).
Sapphire is chemically very stable and is therefore etched little, so that a sapphire substrate 101 is left unetched after the dry etching process. As a result a terraced structure (a terrace portion 103) is formed as extending beyond a cavity mirror 102xe2x80x2 of a multi-layer body 102 formed from group-III nitrides, as shown in FIG. 1. In such a case, a part of a laser light 104 is reflected by the terrace portion 103, thereby a far-field pattern is formed as a plurality of spots in stead of a single spot. Thus, the semiconductor laser device 100 could not be used particularly as a light source for a optical storage device.
To overcome this problem, a method for transferring the multi-layer body onto a substrate which is highly cleavable and cleaving this sample to form the cavity mirror has been proposed. Specifically, the multi-layer body is formed on the sapphire substrate, and this sample is bonded onto a carrier substrate which is highly cleavable. After removing the sapphire substrate from the multi-layer body, the multi-layer body is cleaved along with the carrier substrate, thereby obtaining a laser device with a cleaved facet serving as the cavity mirror.
In the manufacturing process for the conventional semiconductor laser device 100 mentioned above, if there is even a slight misalignment between the carrier substrate 105 and the multi-layer body 102 which are glued together, as shown in FIG. 2, a cleaved facet 105xe2x80x2 of the carrier substrate 105 is partially extended, in the shape of a xe2x80x9cterracexe2x80x9d, beyond a cavity mirror 102xe2x80x2 of the multi-layer body 102. In such a case, even if a dielectric mirror stack is formed on the cavity mirror 102xe2x80x2 by a sputtering technique, sufficiently high reflectivity cannot be obtained because the coating film thickness varies due to the terrace portion 103xe2x80x2. As a result, sufficient optical characteristics as a laser device cannot be achieved. Furthermore, as mentioned earlier, part of the beam is reflected by the terrace portion 103xe2x80x2, resulting in multiplication of the far-field pattern, causing practical problems.
It is therefore an object of the present invention to provide a group-III nitride semiconductor laser device with excellent optical characteristics, and a method for manufacturing the same without steps requiring high assembly precision.